Understanding the Electronic Structure of Isoindigo in Conjugated Systems: A Combined Theoretical and Experimental Approach

Three polymorphs of a new N,N'-dipropylated isoindigo derivative

A newly synthesized N,N'-dipropyl-substituted isoindigo derivative, namely, 1-propyl-3-(1-propyl-1,2-dihydro-2-oxo-3H-indol-3-ylidene)-1,3-dihydro-2H-indol-2-one, C22H22N2O2, was found to have three polymorphic forms (denoted Forms I, II and III) under various crystallization conditions. Crystal structure analysis indicated that Form III had a significantly different molecular conformation from the other two polymorphs. Their different packing arrangements were correlated with differences in the intermolecular interactions. Thermal measurements revealed that Forms I and II are enantiotropically related, and Form II exhibits thermally dynamic behaviour.

Tweaking the Non-Volatile Write-Once-Read-Many-Times (WORM) Memory using Donor-Acceptor Architecture with Isatin as Core Acceptor

Organic memory devices have attracted attention because they promise flexible electronics, low manufacturing costs, and compatibility with large-scale integration. A series of new D-A architectures were synthesized employing different donor groups and the isatin moiety as the acceptor through Suzuki-Miyaura coupling reactions. Strong intramolecular interactions were observed in the synthesized compounds, further corroborated by an optimal bandgap. The SEM investigation confirmed good molecular ordering and superior thin film surface coverage. All the compounds demonstrated notable binary Write-Once-Read-Many-Times (WORM) memory behaviour. The threshold switching voltage for these D-A systems ranged from -0.79 to -2.37 V, with the compound having isobutyl substituent showing the lowest threshold voltage and maximum ON/OFF ratio of 102, thus outperforming others. The combined effects of charge transfer and charge trapping are responsible for the resistive switching mechanism prevailing in these systems. The alterations in D-A molecules that affect molecular packing, thin film morphology, and, finally, the memory performance of the active layer are highlighted in this work.

Sufficient driving force for quinoidal isoindigo-based diradicaloids with tunable diradical characters

Stable organic π-conjugated diradcialoids with tunable diradical characters can profoundly affect emerging technology. Over the past years, great efforts have been devoted to studying the structure-diradical character relationship in diradicaloids. Herein, a series of quinoidal isoindigo (IID) compounds with different attached terminal end groups were designed. Detailed analysis focuses on elucidating the driving force for evoking and enhancing the diradical character in the quinoidal IID systems. The arylene units of the IID core and the bridged aromatic units determine the contribution of the open-shell diradical form in the ground state. Diradical character y0 correlates well with bond length alternation (BLA), the total HOMA, and the total NICS(1)zz, and it is tuned by bridged aromatic units and terminal end groups in symmetric systems. The zwitterionic character weakens the diradical character in asymmetric systems to different extents. This work contributes to the deep understanding of evoking and enhancing the diradical character in quinoidal IID-based diradcialoids, providing useful guidelines to produce new molecules with desirable properties.

Organic Transistors Based on Highly Crystalline Donor-Acceptor π-Conjugated Polymer of Pentathiophene and Diketopyrrolopyrrole

Oligomers and polymers consisting of multiple thiophenes are widely used in organic electronics such as organic transistors and sensors because of their strong electron-donating ability. In this study, a solution to the problem of the poor solubility of polythiophene systems was developed. A novel π-conjugated polymer material, PDPP-5Th, was synthesized by adding the electron acceptor unit, DPP, to the polythiophene system with a long alkyl side chain, which facilitated the solution processing of the material for the preparation of devices. Meanwhile, the presence of the multicarbonyl groups within the DPP molecule facilitated donor-acceptor interactions in the internal chain, which further improved the hole-transport properties of the polythiophene-based material. The weak forces present within the molecules that promoted structural coplanarity were analyzed using theoretical simulations. Furthermore, the grazing incidence wide-angle X-ray scanning (GIWAXS) results indicated that PDPP-5Th features high crystallinity, which is favorable for efficient carrier migration within and between polymer chains. The material showed hole transport properties as high as 0.44 cm2 V-1 s-1 in conductivity testing. Our investigations demonstrate the great potential of this polymer material in the field of optoelectronics.

Incorporation of Diketopyrrolopyrrole into Polythiophene for the Preparation of Organic Polymer Transistors

Polythiophene, as a class of potential electron donor units, is widely used in organic electronics such as transistors. In this work, a novel polymeric material, PDPPTT-FT, was prepared by incorporating the electron acceptor unit into the polythiophene system. The incorporation of the DPP molecule assists in improving the solubility of the material and provides a convenient method for the preparation of field effect transistors via subsequent solution processing. The introduction of fluorine atoms forms a good intramolecular conformational lock, and theoretical calculations show that the structure displays excellent co-planarity and regularity. Grazing incidence wide-angle X-ray (GIWAXS) results indicate that the PDPPTT-FT is highly crystalline, which facilitates carrier migration within and between polymer chains. The hole mobility of this π-conjugated material is as high as 0.30 cm2 V-1 s-1 in organic transistor measurements, demonstrating the great potential of this polymer material in the field of optoelectronics.

Dipyrrolonaphthyridinedione - (still) a mysterious cross-conjugated chromophore

Dipyrrolonaphthyridinediones (DPNDs) entered the chemical world in 2016. This cross-conjugated donor-acceptor skeleton can be prepared in two steps from commercially available reagents in overall yield ≈15-20% (5 mmol scale). DPNDs can be easily and regioselectively halogenated which opens an avenue to numerous derivatives as well as to π-expansion. Although certain synthetic limitations exist, the current derivatization possibilities provided impetus for numerous explorations that use DPNDs. Structural modifications enable bathochromic shift of the emission to deep-red region and reaching the optical brightness 30 000 M-1 cm-1. Intense absorption and strong emission of greenish-yellow light attracted the interest which eventually led to the discovery of their strong two-photon absorption, singlet fission in the crystalline phase and triplet sensitization. Dipyrrolonaphthyridinedione-based twistacenes broadened our knowledge on the influence of twisting angle on the fate of the molecule in the excited state. Collectively, these findings highlight the compatibility of DPNDs with various applications within organic optoelectronics.

Recent Structural Engineering of Polymer Semiconductors Incorporating Hydrogen Bonds

Highly planar, extended π-electron organic conjugated polymers have been increasingly attractive for achieving high-mobility organic semiconductors. In addition to the conventional strategy to construct rigid backbone by covalent bonds, hydrogen bond has been employed extensively to increase the planarity and rigidity of polymer via intramolecular noncovalent interactions. This review provides a general summary of high-mobility semiconducting polymers incorporating hydrogen bonds in field-effect transistors over recent years. The structural engineering of the hydrogen bond-containing building blocks and the discussion of theoretical simulation, microstructural characterization, and device performance are covered. Additionally, the effects of the introduction of hydrogen bond on self-healing, stretchability, chemical sensitivity, and mechanical properties are also discussed. The review aims to help and inspire design of new high-mobility conjugated polymers with superiority of mechanical flexibility by incorporation of hydrogen bond for the application in flexible electronics.

Enhanced Performance of Cyclopentadithiophene-Based Donor-Acceptor-Type Semiconducting Copolymer Transistors Obtained by a Wire Bar-Coating Method

Herein, we report the fabrications of high-performance polymer field-effect transistors (PFETs) with wire bar-coated semiconducting polymer film as an active layer. For an active semiconducting material of the PFETs, we employed cyclopentadithiophene-alt-benzothiadiazole (CDT-BTZ) that is a D-A-type-conjugated copolymer consisting of a repeated electron-donating unit and an electron-accepting unit, and the other two CDT-based D-A-type copolymer analogues are cyclopentadithiophene-alt-fluorinated-benzothiadiazole (CDT-FBTZ) and cyclopentadithiophene-alt-thiadiazolopyridine (CDT-PTZ). The linear field-effect mobility values obtained from the transfer curve of the PFETs fabricated with the spin-coating were 0.04 cm²/Vs, 0.16 cm²/Vs, and 0.31 cm²/Vs, for CDT-BTZ, CDT-FBTZ, and CDT-PTZ, respectively, while the mobility values measured from the PFETs with the wire bar-coated CDT-BTZ film, CDT-FBTZ film, and CDT-PTZ film were 0.16 cm²/Vs, 0.28 cm²/Vs, and 0.95 cm²/Vs, respectively, which are about 2 to 4 times higher values than those of the PFETs with spin-coated films. These results revealed that the aligned molecular chain is beneficial for the D-A-type semiconducting copolymer even though the charge transport in the D-A-type semiconducting copolymer is known to be less critical to the degree of disorder in film.

Predicting the substituent effects in the optical and electrochemical properties of N,N'-substituted isoindigos

Isoindigo, the structural isomer of the well-known dye indigo, has seen a major revival recently because of the increasing interest of its use as a potential drug core structure and for the development of organic photovoltaic materials. Highly beneficial for diverse applications are its facile synthesis, straightforward functionalisation and the broad absorption band in the visible range. Moreover, its intrinsic electron deficiency renders isoindigo a promising acceptor structure in bulk heterojunction architectures. Here we present new insights into the substituent effects of N-functionalised isoindigos, developing a reliable and fast in silico screening approach of a library of compounds. Using experimental UV-Vis and electrochemical data increased the accuracy of the TD-DFT method employed. This procedure allowed us to accurately predict the optical and electrochemical properties of N-functionalised isoindigos and the elucidation of the relationship between substituent effects and electronic properties.

Pd-Catalyzed Direct C-H Activation for the C5-Olefination of Methyleneindolinones

The direct C-H activation without directing groups can realize the para-selectivity, which is a powerful and concise approach for functionalization of arenes. Utilizing the strategy, a C5-olefination of methyleneindolinones has been successfully developed by palladium-catalyzed direct C-H activation, which provides an expeditious access to 5-vinylindolin-2-ones with high regioselectivity. The protocol is distinguished by a mild reaction system avoiding ligand and high temperature. The kinetic isotope experiments indicate that the C-H bond cleavage is the rate-limiting step.

Nitronyl Nitroxide Bifunctionalized Electron-Poor Chromophores: Synthesis of Stable Dye Biradicals by Lewis Acid Promoted Desilylation

Open shell organic molecules bearing π-cores are of great interest for optical, electronic, and magnetic applications but frequently suffer fast decomposition or lack synthetic accessibility. In this regard, nitronyl nitroxides are promising candidates for stable (bi-)radicals due to their high degree of spin delocalization along the O-N-C-N-O pentad unit. Unfortunately, they are limited to electron-rich systems so far. To overcome this limitation, we developed a synthetic procedure for the twofold spin decoration of electron-poor chromophores (E_red = -1158 mV) with nitronyl nitroxide radical moieties via selective deprotection/oxidation of the respective silylated precursors with boron fluoride and subsequent quenching with tetraethyl orthosilicate. Nitronyl nitroxide biradicals PBI-NN, IIn-NN, PhDPP-NN, ThDPP-NN, and FuDPP-NN bridged by perylene bisimide (PBI), isoindigo (IIn), and diketopyrrolopyrrole (DPP) pigment colorants were finally obtained as bench stable compounds after periodate oxidation with yields of 60-81%. The absorption spectral signatures of the chromophores remain preserved in the open shell state and match the ones of the pristine parent compounds, which allowed an a priori prediction of their optical properties. Consequently, we achieved twofold spin labeling while keeping the intrinsic properties of the electron deficient chromophores intact.

Evaluation of Fused Aromatic-Substituted Diketopyrrolopyrrole Derivatives for Singlet Fission Sensitizers

Singlet fission (SF) is a spin-allowed carrier multiplication process that has potential to overcome the Shockley-Queisser limit of solar energy conversion efficiency for single-junction solar cells. It is of importance to prescreen appropriate SF candidates for both basic research and practical applications of SF. Besides common polycyclic aromatic hydrocarbons (PAHs), diketopyrrolopyrrole (DPP) derivatives also undergo efficient SF. A series of DPP derivatives with fused aromatic substituents were investigated considering their conjugation length, constitution, and the introduction of terminal substituents. A comparison of SF properties between nonfused and fused aromatic-substituted DPP derivatives was carried out. Detailed analysis focused on elucidating the relationship between the frontier molecular orbital energies, multiple diradical characters, and SF-relevant excited-state energy levels. Compared to nonfused aromatic-substituted DPP derivatives, fused aromatic-substituted DPP derivatives which contain three aromatic units (thiophene or furan) still share more appropriate energy levels for SF sensitizers. Changing the five-membered aromatic units with benzene ring and introducing -F, -OMe, and -COOH as terminal substituents are both effective ways to improve their performance as SF sensitizers. The results of this research help us to understand the SF properties of DPP derivatives deeply and are beneficial for the design of new DPP-based SF sensitizers.

Polymerization-induced photothermy: A non-donor-acceptor approach to highly effective near-infrared photothermal conversion nanoparticles

Photothermal conversion nanoagents based on conjugated polymers (CPs) are attracting increasing attention for in vivo disease theranostics and high-performing ones are in urgent pursuit. Herein, we report a new and non-donor-acceptor approach to photothermal conversion CPs that combine several merits including low bandgaps, strong near-infrared absorption, low intersystem crossing rate and non-emissive nature. Three CPs based on 6,7; 6',7'-fused isoindigos (nIIDs), i.e., P2IIDV, P3IIDV and P4IIDV that have optical bandgaps of 1.30, 1.22 and 1.17 eV, respectively, are synthesized. The nanoparticles (NPs) of the CPs in water are prepared via nanocoprecipitation, which are non-fluorescent due to the rapid intramolecular twisting in the CP backbone within NPs, enabling most of the excitation energy flow to generate heat. The photothermal conversion efficiencies of the NPs as measured under irradiation at 808, 880 and 980 nm are 62.4%, 40.5% and 15.8% for P2IIDV, 65.1%, 41.0% and 38.9% for P3IIDV and 71.5%, 48.9% and 41.7% for P4IIDV, which are significantly higher than indocyanine green and many popularly reported photothermal conversion materials. In vivo studies using xenograft 4T1 tumor-bearing mouse model demonstrate that the P4IIDV NPs can serve as a rather effective photothermal conversion nanoagent for enhanced photoacoustic imaging and photothermal therapy of tumors.

Synthesis and Characterization of Isoindigo-Based Push-Pull Chromophores

Symmetrical and unsymmetrical chromophores of isoindigo 3-7 were designed and synthesized, in which isoindigo was used as the central unit (electron acceptor unit A), triphenylamine as the end capping unit (electron donor group D), 1,1,4,4-tetracyanobutadiene (TCBD, A') and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (A″) as the acceptor unit. The effects of multiacceptor units on photophysical, electrochemical, and computational studies were investigated. The photophysical properties of isoindigo 6 and 7 exhibit a strong intramolecular charge transfer (ICT) absorption band in the near IR region. The isoindigo 4-7 shows multi-redox waves with a low electrochemical band gap, which signifies the tuning of highest occupied molecular orbital-lowest unoccupied molecular orbital energy levels and enhance the π-conjugation. The computational studies demonstrate that there is a good agreement with experimental data. The molecular design and synthesis of isoindigo 4-7 gives a new avenue for the development of building blocks in organic electronics.

Revealing the Nature of Singlet Fission under the Veil of Internal Conversion

Singlet fission (SF) holds the potential to boost the maximum power conversion efficiency of photovoltaic devices. Internal conversion (IC) has been considered as one of the major competitive deactivation pathways to transform excitation energy into heat. Now, using time-resolved spectroscopy and theoretical calculation, it is demonstrated that, instead of a conventional IC pathway, an unexpected intramolecular singlet fission (iSF) process is responsible for excited state deactivation in isoindigo derivatives. The ¹ TT state could form at ultrafast rate and nearly quantitatively in solution. In solid films, the slipped stacked intermolecular packing of a thiophene-functionalized derivative leads to efficient triplet pair separation, giving rise to an overall triplet yield of 181 %. This work not only enriches the pool of iSF-capable materials, but also contributes to a better understanding of the iSF mechanism, which could be relevant for designing new SF sensitizers.

Synthesis and Density Functional Theory Studies of Azirinyl and Oxiranyl Functionalized Isoindigo and (3Z,3'Z)-3,3'-(ethane-1,2-diylidene)bis(indolin-2-one) Derivatives

The design and synthesis of functionalized isoindigo compounds by reaction of isoindigo with (S)-glycidyl tosylate, epibromohydrin, 2-(bromomethyl)-1-(arylsulfonyl)aziridine, and 2-(bromomethyl)-1-(alkylsulfonyl)aziridine in the presence of MeONa proceed under mild conditions in moderate yields. (3Z,3'Z)-3,3'-(Ethane-1,2-diylidene)bis(1-(oxiran-2-ylmethyl)indolin-2-one), with an extended central olefin π-conjugated moiety was also reacted with methyl-oxiranes to give the corresponding N,N'-disubstituted derivative. Calculations with DFT and TD-DFT of hypothetical isoindigo-thiophene DA molecules with various electron withdrawing substituents, including aziridine, oxirane, nitrile, carbonyl, and sulfonate, indicated that the proximity and strength of the functional group have a significant effect on the HOMO, LUMO, vertical excitation energy, and oscillator strength of the π-π* transitions.

Synthesis and photophysical characterization of isoindigo building blocks as molecular acceptors for organic photovoltaics

Five isoindigo-based donor-acceptor-donor (D-A-D) type small molecules have been synthesized in order to investigate their intramolecular charge transfer characteristics. UV-vis absorption of these dyes exhibits a wide absorption band ranging from 300 to 650 nm with two distinct bands, giving the narrow bandgaps between 1.72 and 1.85 eV. Taking into account their HOMO-LUMO energy levels and bandgaps, isoindigo dyes have been used in the active layer of organic solar cell (OSC) devices. When these small molecule semiconductors were used as acceptors with the donor poly(3-hexylthiophene-2,5-diyl (P3HT) polymer in the inverted OSC devices, the highest power conversion efficiency (PCE) was obtained as 0.10% for pyrene-substituted isoindigo derivative.

Fused Isoindigo Ribbons with Absorption Bands Reaching Near-Infrared

Through fusing isoindigo (IID) units at 6,7;6',7'-positions, a series of new near-infrared (NIR) absorbing and stable ribbon-like conjugated molecules, namely nIIDs in which n represents the number of IID units, have been synthesized. The optical band gaps of the molecules are lowered from 2.03 eV of 1IID to 1.12 eV of 6IID with the increase of the conjugation length. 3IID, 4IID, and 6IID have strong absorption in the NIR region and exhibit photothermal conversion efficiencies of greater than 50 % under laser irradiation at λ=808 nm.

Langmuir-Blodgett Thin Films of Diketopyrrolopyrrole-Based Amphiphiles

We report on two π-conjugated donor-acceptor-donor (D-A-D) molecules of amphiphilic nature, aiming to promote intermolecular ordering and carrier mobility in organic electronic devices. Diketopyrrolopyrrole was selected as the acceptor moiety that was disubstituted with nonpolar and polar functional groups, thereby providing the amphiphilic structures. This structural design resulted in materials with a strong intermolecular order in the solid state, which was confirmed by differential scanning calorimetry and polarized optical microscopy. Langmuir-Blodgett (LB) films of ordered mono- and multilayers were transferred onto glass and silicon substrates, with layer quality, coverage, and intermolecular order controlled by layer compression pressure on the LB trough. Organic field-effect transistors and organic photovoltaics devices with active layers consisting of the amphiphilic conjugated D-A-D-type molecules were constructed to demonstrate that the LB technique is an effective layer-by-layer deposition approach to fabricate self-assembled, ordered thin films.

π-Conjugation expanded isoindigo derivatives and the donor-acceptor conjugated polymers: synthesis and characterization

Two π-conjugation expanded isoindigo derivatives (DIID-PhCO and TIID-PhCO) composed of up to 18 rings and two donor-acceptor conjugated polymers based on DIID-PhCO were synthesized. Both polymers are ambipolar semiconductors with balanced hole and electron mobilities up to 0.10 and 0.14 cm² V^-1 s^-1, respectively, under ambient conditions.

Conjugated Thiophene-Fused Isatin Dyes through Intramolecular Direct Arylation

We report on the design, synthesis, and properties of innovative, planar, π-conjugated compounds in which a thiophene ring is fused with the skeleton of the naturally occurring dye isatin. The synthesis is achieved in high yields making use of an intramolecular direct arylation reaction as the key step, making the overall process potentially scalable. The synthetic sequence has been demonstrated also for an isatin bearing fluorine substituents on the aromatic ring. NMR and X-ray studies demonstrate the crosstalk occurring between the fused, coplanar, and conjugated moieties, making these novel dyes with a donor-acceptor character. Cyclic voltammetry and UV-vis studies confirm very interesting HOMO-LUMO levels and energy gaps for the new compounds.

Influence of structure–property relationships of two structural isomers of thiophene-flanked diazaisoindigo on carrier-transport properties

Two structural isomers of thiophene-flanked diazaisoindigo, 6,6′-substituted 6,6′-T-DAII and 5,5′-substituted 5,5′-T-DAII, have been synthesized to study the influence of the connecting modes on the carrier-transport properties.

A side-chain engineering approach to solvent-resistant semiconducting polymer thin films

Latent H-bonds in conjugated statistical copolymers rendered semiconducting thin films resistant to solvent immersion.

π-Conjugated Organometallic Isoindigo Oligomer and Polymer Chromophores: Singlet and Triplet Excited State Dynamics and Application in Polymer Solar Cells

An isoindigo based π-conjugated oligomer and polymer that contain cyclometalated platinum(II) "auxochrome" units were subjected to photophysical characterization, and application of the polymer in bulk heterojunction polymer solar cells with PCBM acceptor was examined. The objective of the study was to explore the effect of the heavy metal centers on the excited state properties, in particular, intersystem crossing to a triplet (exciton) state, and further how this would influence the performance of the organometallic polymer in solar cells. The materials were characterized by electrochemistry, ground state absorption, emission, and picosecond-nanosecond transient absorption spectroscopy. Electrochemical measurements indicate that the cyclometalated units have a significant impact on the HOMO energy level of the chromophores, but little effect on the LUMO, which is consistent with localization of the LUMO on the isoindigo acceptor unit. Picosecond-nanosecond transient absorption spectroscopy reveals a transient with ∼100 ns lifetime that is assigned to a triplet excited state that is produced by intersystem crossing from a singlet state on a time scale of ∼130 ps. This is the first time that a triplet state has been observed for isoindigo π-conjugated chromophores. The performance of the polymer in bulk heterojunction solar cells was explored with PC61BM as an acceptor. The performance of the cells was optimum at a relatively high PCBM loading (1:6, polymer:PCBM), but the overall efficiency was relatively low with power conversion efficiency (PCE) of 0.22%. Atomic force microscopy of blend films reveals that the length scale of the phase separation decreases with increasing PCBM content, suggesting a reason for the increase in PCE with acceptor loading. Energetic considerations show that the triplet state in the polymer is too low in energy to undergo charge separation with PCBM. Further, due to the relatively low LUMO energy of the polymer, charge transfer from the singlet to PCBM is only weakly exothermic, which is believed to be the reason that the photocurrent efficiency is relatively low.

Synthesis and photovoltaic properties of A–D–A type non-fullerene acceptors containing isoindigo terminal units

Four solution-processable acceptor–donor–acceptor structured organic molecules with isoindigo as terminal acceptor units and different aromatic rigid planar cores as donor units were designed and synthesized as the acceptor materials in organic solar cells (OSCs).

Isoindigo-Containing Molecular Semiconductors: Effect of Backbone Extension on Molecular Organization and Organic Solar Cell Performance

We have synthesized three new isoindigo-based small molecules by extending the conjugated length through the incorporation of octyl-thiophene units between the isoindigo core and benzothiophene terminal units. Both UV-vis and Grazing incidence X-ray diffraction experiments show that such extension of the π-conjugated backbone can induce H-aggregation, and enhance crystallinity and molecular ordering of these isoindigo-based small molecules in the solid state. Compared to two other isoindigo-based derivatives in the series, the derivative with two octyl-thiophene units, BT-T2-ID, is the most crystalline and ordered, and its molecular packing motif appears to be substantially different. Devices utilizing these new extended isoindigo-based small molecules as the electron donor exhibit higher performance than those utilizing nonextended BT-ID as the electron donor. Particularly, devices containing BT-T2-ID in an as-cast blend with PC₆₁BM show power conversion efficiencies up to 3.4%, which is comparable to the best devices containing isoindigo-based molecular semiconductors and is a record among devices containing isoindigo-based small molecules that were processed in the absence of any additives.

Structure-property relationship study of substitution effects on isoindigo-based model compounds as electron donors in organic solar cells

We designed and synthesized a series of isoindigo-based derivatives to investigate how chemical structure modification at both the 6,6'- and 5,5'-positions of the core with electron-rich and electron-poor moieties affect photophysical and redox properties as well as their solid-state organization. Our studies reveal that 6,6'-substitution on the isoindigo core results in a stronger intramolecular charge transfer band due to strong electronic coupling between the 6,6'-substituent and the core, whereas 5,5'-substitution induces a weaker CT band that is more sensitive to the electronic nature of the substituents. In the solid state, 6,6'-derivatives generally form J-aggregates, whereas 5,5'-derivatives form H-aggregates. With only two branched ethylhexyl side chains, the 6,6'-derivatives form organized lamellar structures in the solid state. The incorporation of electron-rich benzothiophene, BT, substituents further enhances ordering, likely because of strong intermolecular donor-acceptor interactions between the BT substituent and the electron-poor isoindigo core on neighboring compounds. Collectively, the enhanced photophysical properties and solid-state organization of the 6,6'-benzothiophene substituted isoindigo derivative compared to the other isoindigo derivatives examined in this study resulted in solar cells with higher power conversion efficiencies when blended with a fullerene derivative.

Panchromatic donor-acceptor-donor conjugated oligomers for dye-sensitized solar cell applications

We report on a sexithienyl and two donor-acceptor-donor oligothiophenes, employing benzothiadiazole and isoindigo as electron-acceptors, each functionalized with a phosphonic acid group for anchoring onto TiO2 substrates as light-harvesting molecules for dye sensitized solar cells (DSSCs). These dyes absorb light to wavelengths as long as 700 nm, as their optical HOMO/LUMO energy gaps are reduced from 2.40 to 1.77 eV with increasing acceptor strength. The oligomers were adsorbed onto mesoporous TiO2 films on fluorine doped tin oxide (FTO)/glass substrates and incorporated into DSSCs, which show AM1.5 power conversion efficiencies (PCEs) ranging between 2.6% and 6.4%. This work demonstrates that the donor-acceptor-donor (D-A-D) molecular structures coupled to phosphonic acid anchoring groups, which have not been used in DSSCs, can lead to high PCEs.

Recent developments on isoindigo-based conjugated polymers

A review of recent advances in isoindigo-based conjugated polymers for organic photovoltaic and field-effect transistor applications is presented.